Magnetic recording systems have several excellent advantages such as allowing for repeated use of magnetic recording mediums easy conversion of recorded information into electric signals, allowing for the magnetic recording system to be combined with peripheral electronic equipment to form a composite system and easy correction of electric signals as compared to other recording systems. Accordingly, the magnetic recording system has been widely used in the fields of video, audio, computer, etc. It has been demanded that the recording density of the recording mediums be further improved to meet the requirements of miniaturization of equipment, improvement in the quality of reproducing signals, prolonging the recording and increasing the recording capacity.
In coated magnetic recording disks, various methods have been proposed such as a method wherein ferromagnetic powder having a finer particle size is used, a method wherein dispersibility is improved, and a method wherein the packing degree of ferromagnetic powder in a magnetic layer is increased. As a further effective means, there is a method using ferromagnetic metallic powder having excellent magnetic characteristics or hexagonal ferrite.
With the popularization of miniature computers and personal computers as OA (Office Automation) equipment, magnetic recording disks for external memory have come into wide use causing the use of magnetic recording disks to increase.
The magnetic recording disks are stored and used under wide environmental conditions, that is, under wide temperature and humidity conditions, and are further used in places where working conditions are dusty.
It is highly demanded that recording density be improved to achieve large-capacity recording. In particular, it is demanded that the disks be miniaturized. The maximum size of conventional acicular ferromagnetic powders must be much smaller than the recording wavelength or recording bit length in order to obtain a magnetic recording disk suitable for high-density recording. At present, acicular ferromagnetic powders having the size of about 0.3 .mu.m are being used allowing for the possibility of a recording wavelength as short as about 1 .mu.m or less.
The size of the acicular ferromagnetic powder must be decreased to obtain a recording medium capable of conducting higher-density recoding. In such acicular ferromagnetic powder, however, the particle has a size as small as 100 .ANG. or less and a volume as small as 1.times.10.sup.-17 cm.sup.3 or less. As a result, problems occur such as a lowering of the magnetic characteristics by thermal agitation and surface effect, and sufficient orientation can not be obtained even when a magnetic field is applied to the magnetic coating.
Ferromagnetic metallic powder has been examined as a possible ferromagnetic powder to be used to obtain a high-density recording. Further, magnetic recording mediums using, as the ferromagnetic powder, hexagonal ferrite particles having a tabular form and an axis of easy magnetization in the direction perpendicular to the crystal face [e.g., those described in JP-A-58-6525 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-58-6526] have been developed in recent years. By providing the particles, the average particle size of the ferromagnetic powder could be decreased to 0.05 .mu.m or smaller and a high-density recording could be possible.
It has been demanded that narrower track width used to make further higher-density recording. The use of ferromagnetic metallic powder or ferromagnetic hexagonal ferrite powder capable of achieving high output and high-density recording has been examined in the field of the magnetic disk to meet these demands. Studies have been made to meet the demands of the miniaturization of the magnetic disk and the improvement of recording density. A thinner magnetic layer and a higher output are demanded to improve high-density recording and overwrite electromagnetic characteristics in particular. With a reduction in the thickness of the magnetic layer, there is a fear that running durability would greatly deteriorate. Further, failure in head touch is caused by high-speed revolution with high-density recording.
The overwriting of recording signals having different magnetic wavelengths is usually necessary in magnetic recording disks, such as floppy disks, for use in computers. Heretofore, only the overwriting of two types of signals having different frequencies wherein one is twice as long as the other, that is, the overwriting of 1f and 2f signals is requested. However, for the popular high-capacity magnetic recording disks of 10M byte, not only is the recording wavelength shortened, but also it is demanded that the overwriting of a plurality of signals in a wider frequency ratio of 3:8, such as a RLL signals, can be made. When signals are used wherein the recording wavelengths are short and the difference in recording frequency is large, the overwriting of a signal having a short recording wavelength on a signal having a long recording wavelength as disclosed in JP-A-58-122623 and JP-A-61-74137 can be suitably achieved only by improving the magnetic characteristics of the magnetic layer.
Namely, in the case of conventional magnetic layers having a thickness of 1.0 .mu.m or more, the line of magnetic force does not reach the depth of the magnetic layer when a signal having a shorter recording wavelength is overwritten onto a previously recorded signal having a longer recording wavelength. Accordingly, the previously recorded signal having a longer wavelength can not be erased.
Further, the gap of recording head is narrowed with the improvement of recording density. Accordingly, sufficient recording in the direction of the depth of the recording medium becomes difficult.
Hence, when the thickness of the magnetic layer is reduced to 1 .mu.m or less to solve the above problems, the magnetic layer is liable to be peeled off, drop-out is caused, running durability cannot be ensured, and reliability is reduced.
Accordingly, the improvement of reproducing output, the ensuring of overwrite characteristics and running durability are important factors to provide a magnetic recording disk capable of coping with the above-described high-density recording.
The charging of the magnetic recording disk during running causes an increase in the number of drop-outs due to the deposition of dust. The error rate caused thereby is a serious problem. To improve the -problem with regard to charging, an additive is usually added to prevent the charging of the magnetic layer. Particularly, the addition of carbon black is most effective and widely used. However, the addition of carbon black in the magnetic recording disks for high-density recording reduces the packing density of the magnetic substance and causes a lowering in output. Hence there is a limit to the amount of carbon black to be added, and it is difficult to prevent charging of the disks.
The aforesaid ferromagnetic hexagonal ferrite powder is low in saturation magnetization amount, and when used, it is difficult to obtain a high output as compared to the use of ferromagnetic Co--Fe.sub.2 O.sub.3 powder and ferromagnetic metallic powder. Accordingly, the packing density of the ferromagnetic hexagonal ferrite powder must be increased to provide a magnetic recording disk having a high output. However, since the ferrite powder is composed of fine particles and has a hexagonal form, dispersibility is poor in comparison with conventional ferromagnetic powders. Accordingly, it is difficult to prevent charging of the disks and a high reproducing output.
Various proposals have been made to meet the requirement for the prevention of charging of the disks, high output and the improvement of durability (see, JP-A-55-55431, JP-A-55-55432, JP-A-55-55433, JP-A-55-55434, JP-A-60-164926, JP-A-55-55436, JP-A-62-38523 and JP-A-62-159337).
Specifically, an interlayer is provided between the magnetic layer and the support. Carbon black and a binder resin are coated as the interlayer, and the magnetic layer is then formed on the interlayer.
This method is effective in improving running durability. In the magnetic recording disks for high-density recording, however, it is impossible to obtain excellent electromagnetic characteristics, that is, high reproducing output and satisfactory overwrite characteristics, while keeping sufficient running durability.
When the magnetic layer is extremely thinned, the thickness of the magnetic layer is reduced to 0.5 .mu.m or less to improve overwrite characteristics, and the following problems result by the use of conventional coating techniques.
(1) It is difficult to coat the magnetic layer having a uniform thickness directly on a non-magnetic support. Further, the magnetic layer is liable to be peeled off.
(2) The interlayer (a non-magnetic layer) is used as a lower layer, and the magnetic layer can be coated on the interlayer. However, when coating is conducted layer at a time, an adhesion problem is caused and there is a possibility that the magnetic layer is peeled off and dropout may result.
Studies have shown that multi-layer coating while the non-magnetic layer and the magnetic layer are still in a wetted state, i.e., a wet-on-wet coating system (described in U.S. Pat. No. 4,844,946) is effective in solving the above-described problems.
However, the above-described problems can not be fully solved merely by using the wet-on-wet coating system. The magnetic disk must also be rotated at a high speed with a reduction in track width and an increase in the recording density as mentioned above. In this case, it is difficult to keep the touch of the magnetic head on the surface of the magnetic disk stably. Namely, it is difficult to maintain the accuracy of the magnetic head on the recording track in order to achieve high-density recording.
It is considered that the touch of the magnetic head can be improved by improving the mechanical characteristics of the magnetic disk. Heretofore, inventions for controlling the stiffness of magnetic tapes have been disclosed in the magnetic tape field. For example, JP-A-58-9214 discloses an embodiment wherein the stiffness of thin metal film type magnetic tapes is controlled to from 0.1 to 10 g.multidot.mm. JP-A-60-261020 and JP-A-61-29417 disclose an embodiment wherein the stiffness of the thin metal film type magnetic tapes and ferromagnetic alloy metal powder-coated tapes is controlled by a loop system measuring method. JP-A-53-66203 discloses an embodiment wherein the stiffness in the crosswise direction (i.e., width detection) of thin metal film type magnetic tapes is kept at a value which is larger than that in the lengthwise direction (i.e., machine direction) thereof.
However, the control of the stiffness of the flexible magnetic disk is not disclosed or suggested in any patent specifications. Accordingly, conditions described in the above-described patent specifications could not be applied to the flexible magnetic disk.
Improvements in devices have been proposed as other means for improving head touch. For example, attempts to improve head mechanisms, such as to raise head pressure, have been made. However, the durability of the magnetic disk is deteriorated, and other problems occur.
An effective method which overcomes these problems has not yet been proposed.
The present invention has been accomplished with the view of solving the problems associated with the prior art.